It is a paradox that "resident" tissue macrophages, in contrast with their precursor cells, are devoid of myeloperoxidase - an enzyme implicated in oxygen-dependent microbicidal activity of polymorphonuclear neutrophil leukocytes. We propose to test the hypothesis that macrophages in culture can acquire functional peroxidase and other granulocyte enzymes if they are exposed to, and ingest, degenerating granulocytes, as they do in an inflammatory focus. We will devise in vitro systems that incrementally approach the environment of an experimentally induced inflammation; and at each stage test the cells for activities that might be attributable to a stimulation evoked by this exposure - in the form of enhanced functional, metabolic and biochemical expressions: phagocytosis, stimulated HMP shunt activity, elaboration of superoxide, secretion of lysosomal hydrolases, etc. Some tumoricidal macrophages exhibit peroxidase activity whose identity, function and origin are entirely unknown. We will examine several examples of cytotoxic macrophages with the purpose of answering these questions. Methodology for the entire program involves electron microscopy, immuno-cytochemistry, microbicidal and enzymatic assays; and, for selected studies, autoradiography and ultrastructural cytochemistry.